Clarify Dokploy build-time Supabase configuration

docs/dokploy-deployment.md

@@ -17,12 +17,22 @@ provider, create a separate application and move the domain after its first
 successful deployment. Configure its build type as **Dockerfile**, with the
 Dockerfile and build context both at the repository root.
 
-Set these Docker build arguments in the Dokploy application:
+In the Dokploy application, open **Environment** and add both values under
+**Build Time Arguments** (not only under the regular runtime environment
+variables):
 
-- `NEXT_PUBLIC_SUPABASE_URL`
-- `NEXT_PUBLIC_SUPABASE_PUBLISHABLE_KEY`
+```text
+NEXT_PUBLIC_SUPABASE_URL=<your Supabase project URL>
+NEXT_PUBLIC_SUPABASE_PUBLISHABLE_KEY=<your Supabase publishable key>
+```
 
-Set the required runtime environment values in Dokploy as well:
+These values are used by Next.js while compiling browser code. Setting them
+only as runtime environment variables does not populate Dockerfile `ARG`
+values, which causes the build error that says they were empty. Both values
+are public client configuration; do not put `SUPABASE_SECRET_KEY` in Build
+Time Arguments.
+
+Also set the following in Dokploy's regular runtime environment:
 
 - `NEXT_PUBLIC_SUPABASE_URL`
 - `NEXT_PUBLIC_SUPABASE_PUBLISHABLE_KEY`